1. Field of the Invention
The present invention relates to an image processing apparatus capable of converting image data according to an amount of applied toner, an image processing method, and a program.
2. Description of the Related Art
A recent electrophotographic image forming apparatus, such as a laser beam printer (LBP), is configured to perform not only monochrome printing but also color printing along with a technological advance of a controller in a host computer or a printer (serving as an image generation unit).
However, unlike a conventional monochrome printing apparatus, a color printing apparatus uses a plurality of color materials (e.g., cyan, magenta, yellow, and black toners) which are overlapped with each other to form a color image. To appropriately fix and transfer a plurality of color materials on a transfer sheet, an amount of color material applied on a transfer sheet (hereinafter, referred to as “application amount”) is generally restricted to a constant value. The application amount is a value assuring a higher-quality image, because the application amount influences a color reproduction range (i.e., color gamut) available for a printed output, or the glossiness of a printed image.
The application amount places various restrictions on engine design. For example, print speed decreases due to increased fixing time, power consumption in fixing processing increases, and process design is changed depending on a type of a transfer sheet according to the application amount. Improving a degree of freedom in the engine design, performing printing on various types of recording media, reducing power consumption, and attaining a higher processing speed are desired. To satisfy these requirements, image forming apparatuses require a technique capable of producing a higher-quality image even when the amount of color material applied to a transfer sheet is reduced to a lower value (hereinafter, referred to as “low application amount”).
Tint and gradation are key factors to assure the image quality realized by the low application amounts to be comparable to that attainable by ordinary application amounts. However, as described above, the color reproduction range realized by the low application amounts is narrow compared to that obtainable by the ordinary application amounts. Preserving ordinary levels of gradation and tint is difficult because the gradation tends to collapse when an ordinary level of tint is preserved and the tint tends to deteriorate when an ordinary level of gradation is preserved.
In general, a color space representing an input image (e.g., a monitor-dependent color space displayed by a monitor) is different from a color reproduction range of an image output apparatus. An image input apparatus generally has a wider color reproduction range. The image output apparatus cannot reproduce one or more colors included in the input image. Therefore, the image output apparatus compresses image data which is present outside of the color reproduction range so that all of the image data are present in the color reproduction range.
Conventionally, the following technique is available to compress the color space representing the input image into the color reproduction range of an image output device while realizing color reproduction appropriately. The technique includes analyzing a color gamut of input image data and converting the color space into a narrow color space capable of representing the color gamut of the input image, which is selectable from a plurality of color spaces prepared beforehand.
The technique further includes performing compressing image data into the color reproduction range of the image output device, using color gamut compression (color gamut mapping) parameters prepared beforehand in the converted color space. The technique can reduce a degree in the color gamut compression and can realize the color reproduction adequately, as discussed in Japanese Patent Application Laid-Open No. 2006-173827.
However, according to the above-described conventional technique, deviation in distribution of input pixels in the selected color space is not taken into consideration. Therefore, the conventional technique may select an excessively wide color space to perform the color gamut compression only for a small number of pixels that do not contribute to quality of an image. Furthermore, the conventional technique requires color gamut compression parameters prepared beforehand for respective color spaces.
Moreover, the color reproduction range of an image output device realized by low application amounts is narrow compared to that realized by ordinary application amounts. Therefore, if the application amount decreases to attain the ordinary tint, the gradation tends to collapse. If the application amount decreases to attain the ordinary gradation, the tint tends to deteriorate. Therefore, it is necessary to provide an appropriate method for compressing a color gamut to obtain a high-quality image.